Many boot and binding combinations, especially in the skiing industry, have a rigid or semi-rigid boot mounted to a binding that is in turn mounted to an athletic device, such as a ski or a snowboard. The rigid boots can surround and secure the users feet providing the needed connection between the feet and the device. In some examples, a similar construction can be used with a snowboard or other applications that can require a secure connection between user and device. In some examples, a wakeboard, surfboard, skateboard, roller skate, ice skate, or other device can also require such a secure connection.
However, a rigid or semi-rigid boot, while providing a responsive connection to the device, often restricts the user from bending his or her feet at the ankle, making the boot uncomfortable and walking difficult. A semi-rigid outer shell surrounding the foot also can make entry into the boots inconvenient for the user.
The closure around the foot of conventional types of footwear can be designed with a tongue positioned internal to the side panels. The tongue can be drawn toward the dorsal surface of the foot and anterior surface of the lower leg via laces or other closure means which alternately cross from one side of the boot to the other, over the outer surface of the tongue. Tightening the laces creates a set of radial closure forces, the vectors of which are often positioned diagonally across the top of the foot. When loosened, the footwear is allowed to open sufficiently for entry and exit of the foot, while closure draws the tongue and side panels together to reduce the interior volume of the footwear.
Laces positioned on the exterior surface of the conventional boot generally are hindered in their closure action by three factors. First, the friction of each lace sliding against the exterior surface of the boot, second, by lace-to-lace contact where two portions of a lace cross and impinge on each other, and third, by the fact that laces are most commonly arranged in such a way that their path over the top of the foot is angular. This angularity of lace pathways naturally increases as the boot's side panels are drawn toward each other. To effectively tighten the boot, both the frictional losses and the decreasing mechanical advantage of diagonally positioned laces must be overcome by the user, either through manual or mechanical means of tensioning the laces. The end result is that more effort is required to achieve a desired degree of closure, and in many cases, a desired level of support cannot be achieved.
In addition to the closure deficiency caused by conventional laces, the common positioning of the boot's tongue inside the side panels of the boot can be problematic. Pressure points against the foot and leg created by the edges of the tongue are a common fitting complaint. And while it is desirable to keep the foot clean and dry in a hostile environment, the dorsal surface of a conventional tongue, lying below the surface of the side panels, encourages snow, water, and contaminants to gather on the outer surface of the tongue and eventually often enter the boot to affect the foot. Robust gusseting and other sealing means are normally employed to avoid intrusion of these elements into the boot, but many users find that leakage eventually takes place after prolonged use.
Ski boots can be designed with an all-encompassing shell of moderately rigid material, typically injection molded in polyurethane or polyether thermoplastic resins. The resulting shell forms a highly supportive structure around the foot and lower leg which allows for a skier's body motions to be transmitted to the skis for turning and control maneuvers. This well-developed state of the art yields good performance but suffers from three main deficiencies:
First, the rigidity of the outer shell imposes fit constraints on the user. The range of closure to accommodate various foot and leg geometries is limited by the lack of flexibility of the shell material. Stiffer shells perform better but at the sacrifice of closure range and comfort. Softer shells sacrifice efficient energy transfer and skiing performance but may accommodate a wider variety of foot shapes.
Second, the rigidly closed structure, which typically extends above the ankle joint, impedes natural walking. With the foot thus enclosed and unable to make use of its normal range of motion, walking and other activities such as climbing stairs are found to be awkward and uncomfortable.
Third, the relatively dense polymer shells of conventional ski boots typically have high heat transfer characteristics which lead to the common complaint of ski boots being cold. Additional insulation added between shell and foot may reduce heat transfer but can create a bulky, heavier boot than desired.